As a technique for removing an acid gas such as CO2 and H2S contained in the gasified gas obtained by gasifying the coal, the biomass or the like in the gasification furnace, conventionally, a chemical absorption method (for example, an amine absorbent (for example, using (absorbent such as N-methyldiethanolamine: MDEA)) and a physical absorption method (for example, using Selexol absorbent using polyethylene glycol dimethyl ether) have been suggested.
Incidentally, in the case of a system such as an integrated coal gasification combined cycle (IGCC) technique, there are demands as follows.
1) In a power generation system, in order to set the discharge of SOx as air pollutants to a level less than a regulation value, it is necessary to remove H2S that is a generation source of SOx. Meanwhile, since there is an effect of enhancing the power generation efficiency, it is desirable that CO2 should not be recovered as much as possible.
2) A small flow rate of recovered H2S-containing gas (off-gas) and a high H2S density are advantageous for the case of manufacturing chemical products from the recovered gas and the case of processing H2S, and it is desirable to be able to selectively recover H2S.
3) In a system that combines the CO shift and CCS (recovery and storage of carbon dioxide) in the IGCC, it is necessary to suppress the H2S density in CO2 recovered by the CO2 recovery process approximately to a specified value (for example, 10 to 20 ppm).
4) In order to improve the power generation efficiency, the amount of use of heat energy such as steam is preferably as small as possible.
That is, it is required to efficiently and selectively separate H2S from the gas containing CO2 and H2S in terms of thermal energy.
Therefore, conventionally, there has been a suggestion of an energy-saving process which supplies a part of the absorbent, in which dissolved components are partly dissipated in a pressure discharge vessel (regenerator upper stage), from the lower part of the uppermost part of the absorber (Patent Literature 1).
The technique of Patent Literature 1 is effective in the case of being applied to the CO2 recovery from the gas containing no H2S. However, in the case of being applied to the selective recovery of H2S from the gas containing CO2 and H2S, the H2S density in the absorbent in the lower part of the absorber increases, and the H2S absorption rate is greatly reduced. Accordingly, there is a problem in that the H2S removal ratio and the H2S selectivity are lowered, and in order to obtain the desired removal ratio, on the contrary, an increase in thermal energy is caused.
Thus, the inventors have previously suggested a technique which extracts a part of the absorbent from the middle of the absorption portion of the absorber and supplies the absorbent, which absorbs CO2 and H2S at a relatively low density, to the middle of a regeneration portion of the regenerator (Patent Literature 2).